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| United States Patent |
5,771,211
|
|
Tanase
, et al.
|
June 23, 1998
|
Magneto-optical recording media having a reading layer with a specified
range of temperature coefficients of a kerr rotation angle
Abstract
The magneto-optical recording medium, including a transparent polycarbonate
substrate, an interference layer made of SiN, a reading layer made of
GdFeCo, a recording layer made of TbFeCo, an oxidation-preventing layer
made of SiN, a irradiation layer made of Al and an ultraviolet-setting
plastic layer, is disclosed. Each layer is deposited to a suitable
thickness in the above-named order on the transparent polycarbonate
substrate. The composition of each element in the reading layer and the
recording layer is set to a suitable value to achieve acceptable
characteristics. A high recording and reading density is achieved.
| Inventors:
|
Tanase; Kenji (Gifu, JP);
Suzuki; Yoshihisa (Bisai, JP);
Yamaguchi; Atsushi (Oogaki, JP)
|
| Assignee:
|
Sanyo Electric Co., Ltd. (Osaka, JP)
|
| Appl. No.:
|
639365 |
| Filed:
|
April 26, 1996 |
Foreign Application Priority Data
| Apr 26, 1995[JP] | 7-127095 |
| Aug 31, 1995[JP] | 7-224387 |
| Nov 22, 1995[JP] | 7-304345 |
| Nov 24, 1995[JP] | 7-329915 |
| Nov 30, 1995[JP] | 7-313148 |
| Current U.S. Class: |
369/13.5; 428/820.4 |
| Intern'l Class: |
G11B 011/00 |
| Field of Search: |
369/13,14,275.2
360/59,114
428/694 ML,694 SC,694 MT,694 MM,694 EC
365/122
|
References Cited
U.S. Patent Documents
| 5278810 | Jan., 1994 | Takahashi et al. | 369/13.
|
| 5486395 | Jan., 1996 | Murakami et al. | 369/13.
|
| 5563852 | Oct., 1996 | Murakami et al. | 369/13.
|
| Foreign Patent Documents |
| 586175 | Mar., 1994 | EP.
| |
| 596716 | May., 1994 | EP.
| |
| 608134 | Jul., 1994 | EP.
| |
| 606498 | Jul., 1994 | EP.
| |
| 621592 | Oct., 1994 | EP.
| |
| 657880 | Jun., 1995 | EP.
| |
| 668586 | Aug., 1995 | EP.
| |
| 5255393 | Apr., 1995 | JP.
| |
| 7201089 | Aug., 1995 | JP.
| |
| 8147779 | Jun., 1996 | JP.
| |
Primary Examiner: Dinh; Tan
Attorney, Agent or Firm: Loeb & Loeb LLP
Claims
We claim:
1. A magneto-optical recording medium comprising:
a transparent substrate;
a ground layer formed on the transparent substrate;
a reading layer including an in-plane magnetization film, having
substantially in-plane magnetization at room temperature, formed on the
ground layer; and
a recording layer formed on the reading layer for copying a direction of
magnetization into the reading layer by being heated to an inherent
copying temperature of said reading layer, wherein a temperature
coefficient of a Kerr rotation angle of said reading layer is at least
8.0.
2. The magneto-optical recording medium according to claim 1, wherein the
recording layer includes a perpendicular magnetization film having
substantially perpendicular magnetization at room temperature.
3. The magneto-optical recording medium according to claim 1, wherein the
reading layer includes transition metals including Co and a rare-earth
element and an atomic percent of Co in the reading layer is within a range
of 12-50 at %.
4. The magneto-optical recording medium according to claim 2, wherein the
reading layer includes transition metals including Co and a rare-earth
element and an atomic percent of Co in the reading layer is within a range
of 12-50 at %.
5. The magneto-optical recording medium according to claim 3, wherein the
reading layer includes Gd and Fe.
6. The magneto-optical recording medium according to claim 4, wherein the
reading layer includes Gd and Fe.
7. The magneto-optical recording medium according to claim 3, wherein the
ground layer is formed by depositing SiN as an interference layer having a
thickness within a range of 600-800 .ANG..
8. The magneto-optical recording medium according to claim 4, wherein the
ground layer is formed by depositing SiN as an interference layer having a
thickness within a range of 600-800 .ANG..
9. The magneto-optical recording medium according to claim 5, wherein the
ground layer is formed by depositing SiN as an interference layer having a
thickness within a range of 600-800 .ANG..
10. The magneto-optical recording medium according to claim 6, wherein the
ground layer is formed by depositing SiN as an interference layer having a
thickness within a range of 600-800 .ANG..
11. The magneto-optical recording medium according to claim 7, wherein a
thickness of the reading layer is within a range of 800-1200 .ANG..
12. The magneto-optical recording medium according to claim 8, wherein a
thickness of the reading layer is within a range of 800-1200 .ANG..
13. The magneto-optical recording medium according to claim 9, wherein a
thickness of the reading layer is within a range of 800-1200 .ANG..
14. The magneto-optical recording medium according to claim 10, wherein a
thickness of the reading layer is within a range of 800-1200 .ANG..
15. The magneto-optical recording medium according to claim 3, wherein the
recording layer is formed by depositing TbFeCo having a Co atomic percent
within a range of 10-16 at %.
16. The magneto-optical recording medium according to claim 4, wherein the
recording layer is formed by depositing TbFeCo having a Co atomic percent
within a range of 10-16 at %.
17. The magneto-optical recording medium according to claim 5, wherein the
recording layer is formed by depositing TbFeCo having a Co atomic percent
within a range of 10-16 at %.
18. The magneto-optical recording medium according to claim 6, wherein the
recording layer is formed by depositing TbFeCo having a Co atomic percent
within a range of 10-16 at %.
19. The magneto-optical recording medium according to claim 7, wherein the
recording layer is formed by depositing TbFeCo having a Co atomic percent
within a range of 10-16 at %.
20. The magneto-optical recording medium according to claim 8, wherein the
recording layer is formed by depositing TbFeCo having a Co atomic percent
within a range of 10-16 at %.
21. The magneto-optical recording medium according to claim 9, wherein the
recording layer is formed by depositing TbFeCo having a Co atomic percent
within a range of 10-16 at %.
22. The magneto-optical recording medium according to claim 10, wherein the
recording layer is formed by depositing TbFeCo having a Co atomic percent
within a range of 10-16 at %.
23. The magneto-optical recording medium according to claim 11, wherein the
recording layer is formed by depositing TbFeCo having a Co atomic percent
within a range of 10-16 at %.
24. The magneto-optical recording medium according to claim 12, wherein the
recording layer is formed by depositing TbFeCo having a Co atomic percent
within a range of 10-16 at %.
25. The magneto-optical recording medium according to claim 13, wherein the
recording layer is formed by depositing TbFeCo having a Co atomic percent
within a range of 10-16 at %.
26. The magneto-optical recording medium according to claim 14, wherein the
recording layer is formed by depositing TbFeCo having a Co atomic percent
within a range of 10-16 at %.
27. The magneto-optical recording medium according to claim 3, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
28. The magneto-optical recording medium according to claim 4, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
29. The magneto-optical recording medium according to claim 5, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
30. The magneto-optical recording medium according to claim 6, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
31. The magneto-optical recording medium according to claim 7, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
32. The magneto-optical recording medium according to claim 8, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
33. The magneto-optical recording medium according to claim 9, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
34. The magneto-optical recording medium according to claim 10, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
35. The magneto-optical recording medium according to claim 11, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
36. The magneto-optical recording medium according to claim 12, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
37. The magneto-optical recording medium according to claim 13, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
38. The magneto-optical recording medium according to claim 14, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
39. The magneto-optical recording medium according to claim 15, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
40. The magneto-optical recording medium according to claim 16, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
41. The magneto-optical recording medium according to claim 17, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
42. The magneto-optical recording medium according to claim 18, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
43. The magneto-optical recording medium according to claim 19, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
44. The magneto-optical recording medium according to claim 20, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
45. The magneto-optical recording medium according to claim 21, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
46. The magneto-optical recording medium according to claim 22, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
47. The magneto-optical recording medium according to claim 23, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
48. The magneto-optical recording medium according to claim 24, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
49. The magneto-optical recording medium according to claim 25, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
50. The magneto-optical recording medium according to claim 26, wherein the
reading layer is formed by depositing GdFeCo having a Gd atomic percent
within a range of 30-36 at %.
51. The magneto-optical recording medium according to claim 7, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
52. The magneto-optical recording medium according to claim 8, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
53. The magneto-optical recording medium according to claim 9, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of
0.02-0.08W/cm.sup.2.
54. The magneto-optical recording medium according to claim 10, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
55. The magneto-optical recording medium according to claim 11, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
56. The magneto-optical recording medium according to claim 12, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
57. The magneto-optical recording medium according to claim 13, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
58. The magneto-optical recording medium according to claim 14, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
59. The magneto-optical recording medium according to claim 15, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
60. The magneto-optical recording medium according to claim 16, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
61. The magneto-optical recording medium according to claim 17, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
62. The magneto-optical recording medium according to claim 18, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
63. The magneto-optical recording medium according to claim 19, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
64. The magneto-optical recording medium according to claim 20, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
65. The magneto-optical recording medium according to claim 21, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
66. The magneto-optical recording medium according to claim 22, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08 W/cm.
67. The magneto-optical recording medium according to claim 23, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
68. The magneto-optical recording medium according to claim 24, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
69. The magneto-optical recording medium according to claim 25, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
70. The magneto-optical recording medium according to claim 26, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
71. The magneto-optical recording medium according to claim 27, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
72. The magneto-optical recording medium according to claim 28, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
73. The magneto-optical recording medium according to claim 29, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
74. The magneto-optical recording medium according to claim 30, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
75. The magneto-optical recording medium according to claim 31, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
76. The magneto-optical recording medium according to claim 32, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
77. The magneto-optical recording medium according to claim 33, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
78. The magneto-optical recording medium according to claim 34, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
79. The magneto-optical recording medium according to claim 35, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
80. The magneto-optical recording medium according to claim 36, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
81. The magneto-optical recording medium according to claim 37, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
82. The magneto-optical recording medium according to claim 38, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
83. The magneto-optical recording medium according to claim 39, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
84. The magneto-optical recording medium according to claim 40, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
85. The magneto-optical recording medium according to claim 41, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
86. The magneto-optical recording medium according to claim 42, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
87. The magneto-optical recording medium according to claim 43, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
88. The magneto-optical recording medium according to claim 44, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
89. The magneto-optical recording medium according to claim 45, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
90. The magneto-optical recording medium according to claim 46, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
91. The magneto-optical recording medium according to claim 47, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
92. The magneto-optical recording medium according to claim 48, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
93. The magneto-optical recording medium according to claim 49, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
94. The magneto-optical recording medium according to claim 50, wherein the
reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
95. The magneto-optical recording medium according to claim 27, wherein the
reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
96. The magneto-optical recording medium according to claim 28, wherein the
reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
97. The magneto-optical recording medium according to claim 29, wherein the
reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
98. The magneto-optical recording medium according to claim 30, wherein the
reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
99. The magneto-optical recording medium according to claim 31, wherein the
reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
100. The magneto-optical recording medium according to claim 32, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
101. The magneto-optical recording medium according to claim 33, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
102. The magneto-optical recording medium according to claim 34, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
103. The magneto-optical recording medium according to claim 35, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
104. The magneto-optical recording medium according to claim 36, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
105. The magneto-optical recording medium according to claim 37, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
106. The magneto-optical recording medium according to claim 38, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
107. The magneto-optical recording medium according to claim 39, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
108. The magneto-optical recording medium according to claim 40, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
109. The magneto-optical recording medium according to claim 41, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
110. The magneto-optical recording medium according to claim 42, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
111. The magneto-optical recording medium according to claim 43, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
112. The magneto-optical recording medium according to claim 44, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
113. The magneto-optical recording medium according to claim 45, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
114. The magneto-optical recording medium according to claim 46, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
115. The magneto-optical recording medium according to claim 47, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
116. The magneto-optical recording medium according to claim 48, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
117. The magneto-optical recording medium according to claim 49, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
118. The magneto-optical recording medium according to claim 50, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
119. A magneto-optical recording medium comprising:
a transparent substrate made of polycarbonate;
a ground layer formed on the transparent substrate;
a reading layer including an in-plane magnetization film, having
substantially in-plane magnetization at room temperature, formed on the
ground layer; and
a recording layer formed on the reading layer for copying a direction of
magnetization into the reading layer by being heated to an inherent
copying temperature of said reading layer, wherein a temperature
coefficient of a Kerr rotation angle of said reading layer is at least
8.0.
120. The magneto-optical recording medium according to claim 119, wherein
the recording layer includes a perpendicular magnetization film having
substantially perpendicular magnetization at room temperature, the reading
layer includes transition metals including Co and a rare-earth element and
an atomic percent of Co in the reading layer is within a range of 12-50 at
%, and the ground layer is formed by depositing SiN as an interference
layer having a thickness within a range of 600-800 .ANG..
121. The magneto-optical recording medium according to claim 120, wherein a
thickness of the reading layer is within a range of 800-1200 .ANG..
122. The magneto-optical recording medium according to claim 120, wherein
the recording layer is formed by depositing TbFeCo having a Co atomic
percent within a range of 10-16 at %.
123. The magneto-optical recording medium according to claim 121, wherein
the recording layer is formed by depositing TbFeCo having a Co atomic
percent within a range of 10-16 at %.
124. The magneto-optical recording medium according to claim 120, wherein
the reading layer is formed by depositing GdFeCo having a Gd atomic
percent within a range of 30-36 at %.
125. The magneto-optical recording medium according to claim 121, wherein
the reading layer is formed by depositing GdFeCo having a Gd atomic
percent within a range of 30-36 at %.
126. The magneto-optical recording medium according to claim 122, wherein
the reading layer is formed by depositing GdFeCo having a Gd atomic
percent within a range of 30-36 at %.
127. The magneto-optical recording medium according to claim 123, wherein
the reading layer is formed by depositing GdFeCo having a Gd atomic
percent within a range of 30-36 at %.
128. The magneto-optical recording medium according to claim 120, wherein
the reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
129. The magneto-optical recording medium according to claim 121, wherein
the reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
130. The magneto-optical recording medium according to claim 122, wherein
the reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
131. The magneto-optical recording medium according to claim 123, wherein
the reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
132. The magneto-optical recording medium according to claim 124, wherein
the reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
133. The magneto-optical recording medium according to claim 125, wherein
the reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
134. The magneto-optical recording medium according to claim 126, wherein
the reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
135. The magneto-optical recording medium according to claim 127, wherein
the reading layer is formed after a surface of the interference layer is
etched by an etching power intensity within a range of 0.02-0.08
W/cm.sup.2.
136. The magneto-optical recording medium according to claim 124, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
137. The magneto-optical recording medium according to claim 125, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
138. The magneto-optical recording medium according to claim 126, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
139. The magneto-optical recording medium according to claim 127, wherein
the reading layer is formed by sputtering within an atmosphere having a
sputtering gas pressure within a range of 2-7 mTorr.
140. The magneto-optical recording medium according to claim 120, wherein
the recording layer includes TbFeCo, and the reading layer includes a
material selected from the group consisting of GdFeCo, GdFeCoCr, GdFeCoNi,
GdFeCoTi, GdFeCoAl, GdFeCoMn, GdFeCoNiCr, and GdFeCoAlTi.
141. The magneto-optical recording medium according to claim 121, wherein
the recording layer includes TbFeCo, and the reading layer includes a
material selected from the group consisting of GdFeCo, GdFeCoCr, GdFeCoNi,
GdFeCoTi, GdFeCoAl, GdFeCoMn, GdFeCoNiCr, and GdFeCoAlTi.
142. A magneto-optical recording medium comprising:
a transparent substrate made of polycarbonate;
a ground layer formed on the transparent substrate;
a reading layer including an in-plane magnetization film, having
substantially in-plane magnetization at room temperature, formed on the
ground layer; and
a recording layer formed on the reading layer for copying a direction of
magnetization into the reading layer by being heated to an inherent
copying temperature of said reading layer; and
a radiation layer formed on the recording layer, wherein a temperature
coefficient of a Kerr rotation angle of said reading layer is at least
8.0.
143. The magneto-optical recording medium according to claim 142, wherein
the recording layer includes a perpendicular magnetization film having
substantially perpendicular magnetization at room temperature, the reading
layer includes transition metals including Co and a rare-earth element and
an atomic percent of Co in the reading layer is within a range of 12-50 at
%, and the ground layer is formed by depositing SiN as an interference
layer having a thickness within a range of 600-800 .ANG..
144. The magneto-optical recording medium according to claim 143, wherein a
thickness of the reading layer is within a range of 800-1200 .ANG..
145. The magneto-optical recording medium according to claim 143, wherein
the recording layer is formed by depositing TbFeCo having a Co atomic
percent within a range of 10-16 at %.
146. The magneto-optical recording medium according to claim 144, wherein
the recording layer is formed by depositing TbFeCo having a Co atomic
percent within a range of 10-16 at %.
147. The magneto-optical recording medium according to claim 143, wherein
the reading layer is formed by depositing GdFeCo having a Gd atomic
percent within a range of 30-36 at %.
148. The magneto-optical recording medium according to claim 144, wherein
the reading layer is formed by depositing GdFeCo having a Gd atomic
percent within a range of 30-36 at %.
149. The magneto-optical recording medium according to claim 145, wherein
the reading layer is formed by depositing GdFeCo having a Gd atomic
percent within a range of 30-36 at %.
150. The magneto-optical recording medium according to claim 146, wherein
the reading layer is formed by depositing GdFeCo having a Gd atomic
percent within a range of 30-36 at %.
151. The magneto-optical recording medium according to claim 143, wherein
the recording layer includes TbFeCo, and the reading layer includes a
material selected from the group consisting of GdFeCo, GdFeCoCr, GdFeCoNi,
GdFeCoTi, GdFeCoAl, GdFeCoMn, GdFeCoNiCr, and GdFeCoAlTi.
152. The magneto-optical recording medium according to claim 142, wherein
the radiation layer includes at least one metal selected from the group
consisting of Al, Au, Pt, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ag, Sn,
Sb and W.
153. The magneto-optical recording medium according to claim 143, wherein
the radiation layer includes at least one metal selected from the group
consisting of Al, Au, Pt, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ag, Sn,
Sb and W.
154. The magneto-optical recording medium according to claim 151, wherein
the radiation layer includes at least one metal selected from the group
consisting of Al, Au, Pt, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ag, Sn,
Sb and W.
155. The magneto-optical recording medium according to claim 142, wherein a
thickness of the radiation layer is within a range of 200-1000 .ANG..
156. The magneto-optical recording medium according to claim 152, wherein a
thickness of the radiation layer is within a range of 200-1000 .ANG..
157. The magneto-optical recording medium according to claim 153, wherein a
thickness of the radiation layer is within a range of 200-1000 .ANG..
158. The magneto-optical recording medium according to claim 154, wherein a
thickness of the radiation layer is within a range of 200-1000 .ANG..
159. The magneto-optical recording medium according to claim 155, wherein
the reading layer is formed after a surface of the ground layer is etched
by an etching power intensity within a range of 0.02-0.08 W/cm.sup.2.
160. The magneto-optical recording medium according to claim 152, wherein
the radiation layer is formed by an RF magnetron sputtering method having
a power within a range of 100-1000 W and under an Ar gas pressure within a
range of 1-10 mTorr.
161. The magneto-optical recording medium according to claim 153, wherein
the radiation layer is formed by an RF magnetron sputtering method having
a power within a range of 100-1000 W and under an Ar gas pressure within a
range of 1-10 mTorr.
162. The magneto-optical recording medium according to claim 154, wherein
the radiation layer is formed by an RF magnetron sputtering method having
a power within a range of 100-1000 W and under an Ar gas pressure within a
range of 1-10 mTorr.
163. The magneto-optical recording medium according to claim 142, wherein
the transparent substrate has a double refraction within a range of 20-25
nm, a circumferential variation in the double refraction within a range of
6-10 nm, and a radius of curvature at a corner of a groove and land within
a range of 35-50 nm.
164. The magneto-optical recording medium according to claim 142, wherein
the transparent substrate has a surface roughness within a range of
100-500 .ANG..
165. The magneto-optical recording medium according to claim 1, wherein the
recording layer includes a magnetic layer having a transition point from
antiferromagnetism to ferromagnetism at a temperature of at least
50.degree. C.
166. The magneto-optical recording medium according to claim 165, wherein
the recording layer includes (Mn.sub.(100-x) M.sub.x).sub.2 Sb, where M is
a metal selected from the group consisting of Cr, V, Co, Cu, Zn, Ge and
As.
167. The magneto-optical recording medium according to claim 166, wherein M
is Cr, and atomic percent x is in a range within 10-30 at %.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to magneto-optical recording media
and recording and reading methods for such media. More particularly, the
present invention relates to a magneto-optical recording medium having an
exchange-coupled magnetic layer including a recording layer, which is a
perpendicular magnetization film, and a reading layer, which is an
in-plane magnetization film at about room temperature, and has achieved a
high recording density by being so arranged that the direction of
magnetization of the recording layer is copied into the reading layer when
a recorded signal is read from the medium.
2. Description of the Related Art
The magneto-optical recording medium has drawn attention as a recording
medium which is rewritable, large in storage capacity and high in
reliability. For this reason, this medium is used as a computer storage
medium, for example. However, due to the increase in information volume
and the downsizing of related hardware, it is desirable for this medium to
have a higher recording and reading density.
High density recording and reading technology consists of technology on the
hardware side and technology on the medium side. The hardware side
includes a technique for utilizing optical super-resolution phenomena to
stop down the spot diameter of the laser beam to below the diffraction
limits of the laser beam and a technique for manufacturing the laser diode
which outputs a laser beam of shorter wavelength. The medium side includes
a technique for forming tracks of narrow pitch on the recording medium and
a technique for improving the resolution of reading by utilizing a
magnetic multilayer film.
The technique for improving the resolution of reading by utilizing a
magnetic multilayer film is based on the fact that the temperature
distribution of a laser spot is the most concentrated at and around the
center of the laser spot, constituting a Gaussian distribution. By
utilizing this fact, the state of the recording layer at and around the
center of the laser spot is copied into the reading layer, and then the
state of the reading layer is read.
In the magneto-optical recording medium utilized in conventional optical
super-resolution techniques, generally a recording layer which is a
perpendicular magnetization film is utilized. As a substrate for the
conventional magneto-optical recording medium, generally a glass substrate
is used.
The recording of a signal into the magneto-optical recording medium having
a recording layer and a reading layer is achieved by raising the
temperature of a target domain of the reading layer to the Curie
temperature thereof or higher by focusing a laser beam spot onto the
domain, then lowering the temperature of that domain, and aligning the
direction of magnetization of that domain with the direction of the
applied external magnetic field, and then further lowering the temperature
of that domain to copy the direction of magnetization of that domain into
an corresponding domain of the recording layer. This means that the
recording is performed by a thermal magnetic method, and therefore the
magnetic-temperature characteristics and composition of each layer have an
effect on the recording characteristics. At about room temperature, the
magnetization of the reading layer has an in-plane direction.
In order to raise the temperature of the target domain of the reading layer
to the Curie temperature or higher, the laser power needs to be increased.
If the heating is insufficient, the carrier-to-noise ratio (CNR) of the
recording signal degrades. On the other hand, in order to align the
direction of magnetization of the reading layer whose heating is
insufficient with the direction of the external magnetic field, a strong
magnetic field needs to be applied. However, even if a strong magnetic
field is applied, it is impossible to prevent the degradation in the CNR
of the recording signal. Furthermore, in the magnetic modulation-type
recording, a small magnetic field should preferably be applied.
If the Curie temperature of the recording layer is so low that there is a
large difference between the Curie temperature of the reading layer and
the Curie temperature of the recording layer, in the temperature lowering
process of the recording, when the temperature becomes lower than the
Curie temperature of the recording layer and the direction of the
magnetization of the reading layer begins to be copied to the recording
layer, a part of magnetization of the reading layer has already begun to
take the in-plane direction. This causes the noise of the signal to be
copied from the reading layer into the recording layer, and therefore this
creates a problem in that the CNR of the recording signal decreases.
The reproduction of the information from the magneto-optical recording
medium having a recording layer and a reading layer is based on the fact
that the temperature distribution of a laser spot is the most concentrated
at and around the center of the laser spot, constituting a Gaussian
distribution. By utilizing this fact, the state of the recording layer at
the center of the laser spot is copied into the reading layer, and the
state of the reading later is read. In such reading method, the
temperature distribution of the reading layer whose temperature is raised
by the laser spot should be in a desired distribution. This is because if
such temperature distribution fails to be the desired distribution, noise
caused by disordered directions of magnetization or crosstalk noise,
caused by excessive reading from the circumferential low temperature area
of the center of the laser spot, increases.
In a magneto-optical recording medium of laser passing-type in which the
laser beam passes through the magnetic layer, the heat accumulation caused
by the laser beam is negligible. However, in a magneto-optical recording
medium of laser reflecting type in which the laser beam is reflected by
the magnetic layer, the thickness of the magnetic layer is 400 .ANG. or
more, for example, and the accumulated heat has an effect on the
temperature distribution of the reading layer. Therefore, this creates a
problem in that such noise as described above increases.
Furthermore, as recording in the conventional magneto-optical recording
medium is made by irradiating a laser beam of a certain intensity, the
temperature rise area of the recording layer is larger than the diameter
of the laser spot. As a result, this creates a problem in that the
recording spot is so large that increasing the density is difficult.
In addition, when a glass substrate is used as a substrate, there are other
problems in that the weight of the magneto-optical recording medium is
relatively heavy, the magneto-optical recording medium may be damaged when
it is dropped, the magneto-optical recording medium is not suitable to a
high-speed revolution, the necessity of surface polishing raises the
manufacturing cost, and a guide groove for use in tracking a laser beam
cannot easily be directly formed direct, to name a few.
Moreover, according to conventional methods, such as the conventional CAD
method, the change from an in-plane magnetization film into a
perpendicular magnetization film within the reading layer occurs within a
wide range of temperatures from about several tens of degrees centigrade
(.degree.C.) to near about 100.degree. C., and the magnetization of the
recording layer affects the reading layer so as to disturb the in-plane
magnetization of the reading layer, and the mask effect is degraded.
Therefore, the copying area is not clear, reading noise is large, and the
magnetic super-resolution or MSR effect cannot be achieved as much.
Furthermore, as there is no clear threshold value for copying, the copying
temperature is likely to depend on the material manufacturing conditions.
Therefore, uniform characteristics cannot be obtained.
SUMMARY OF THE INVENTION
It is an object of the present invention for a magneto-optical recording
medium of such type that detects reflected laser beam from the magnetic
layer to reduce the noise described above by making the heat accumulation
negligible.
It is another object of the present invention to provide a magneto-optical
recording medium which can record even if the application magnetic field
is small and can suitably be used for the magnetic field modulation mode.
It is still another object of the present invention to provide a
magneto-optical recording medium which can record with an acceptable
carrier-to-noise ratio or CNR.
It is also an object of the present invention to provide a magneto-optical
recording medium which can achieve high-density recording and reading by
limiting the area which is heated to a certain temperature in recording
and reading.
It is also an object of the present invention to provide a magneto-optical
recording medium which can simply and easily be handled.
It is also an object of the present invention to provide a magneto-optical
recording medium which has a clear copying temperature, a low reading
noise, a large MSR effect and a high uniformity by providing the magnetic
copying function not to the reading layer but to the recording layer
itself.
These objects can be achieved by forming a magneto-optical recording medium
according to the present invention to comprise a transparent substrate 41,
an interference layer 42, a reading layer 43, a recording layer 44, a
protection layer 45, a radiation layer 46 and an ultraviolet-setting
plastic layer 47, as shown in FIG. 17, and designing each of these layers
to have improved characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross-sectional view of a magneto-optical recording
medium according to the first and second examples;
FIG. 2 is a graph illustrating the remanence Kerr rotation angle of the
recording layer and reading layer of the magneto-optical recording medium
of the first example with temperature taken as the abscissa;
FIG. 3 is a graph illustrating the relations between laser power and noise
when the magneto-optical recording medium of the first example is
recording;
FIG. 4 is a graph illustrating the relations between the external magnetic
field and noise when the magneto-optical recording medium of the first
example is recording;
FIG. 5 is a graph illustrating the remanence Kerr rotation angle of the
recording layer and reading layer of the magneto-optical recording medium
of the second example with temperature taken as the abscissa;
FIG. 6 is a graph illustrating the relations between external magnetic
field and CNR when the magneto-optical recording medium of the first
example ›B! and a conventional magnetic recording medium ›A! are
recording;
FIG. 7 is a schematic cross-sectional view of a magneto-optical recording
medium according to the third example;
FIG. 8 is a graph illustrating the CNR of the reading signal of the
magneto-optical recording medium according to the third examples;
FIG. 9 is a schematic cross-sectional view of a magneto-optical recording
medium according to the fourth example;
FIG. 10 is a schematic cross-sectional view of a magneto-optical recording
medium according to the fifth example;
FIG. 11 is a schematic cross-sectional view of a magneto-optical recording
medium according to the sixth through fourteenth examples;
FIG. 12 is a graph illustrating the remanence Kerr rotation angle of the
recording layer and reading layer of the magneto-optical recording medium
of the sixth example compared with a conventional magneto-optical
recording medium with temperature taken as the abscissa;
FIG. 13 shows graphs illustrating the Kerr loops near the vicinity of the
Curie temperature of the magneto-optical recording medium according to the
sixth example compared with a conventional magneto-optical medium;
FIG. 14 is a graph illustrating the magnetic modulation recordings
according to the sixth example compared with the prior art;
FIG. 15 shows plan views of the surfaces of interference layers of the
sixth and seventh examples obtained through an interatomic force
microscope;
FIG. 16 is a graph illustrating the CNR of the reading signal for the
external magnetic field according to the sixth and seventh examples;
FIG. 17 is a schematic cross-sectional view of a magneto-optical recording
medium according to a preferred embodiment of the present invention;
FIG. 18 is a process flow chart of the magneto-optical recording medium
according to a preferred embodiment of the present invention;
FIG. 19 is a view illustrating an injection molding machine for a
transparent polycarbonate substrate of the magneto-optical recording
medium according to a preferred embodiment of the present invention;
FIG. 20 is a chart illustrating the relations between the modulated
external magnetic field and a pulsed laser in the magneto-optical
recording medium according to a preferred embodiment of the present
invention;
FIG. 21 is a block diagram illustrating a recording circuit for
magneto-optical recording medium according to a preferred embodiment of
the present invention;
FIG. 22 is a graph qualitatively illustrating the effect of forming the
radiation layer according to embodiments of the present invention and
utilizing a pulsed laser according to embodiments of the present
invention;
FIG. 23 is a graph illustrating the relations between the phase difference
(between a pulsed magnetic field and a pulsed laser) when the
magneto-optical recording medium according to the examples of the present
invention is recording and the CNR when the same is reading;
FIG. 24 is a graph illustrating the relations between the phase difference
(between a pulsed magnetic field and a pulsed laser) when the
magneto-optical recording medium according to the examples of the present
invention is recording and the CNR when the same is reading;
FIG. 25 is a graph illustrating the relations between the CNR and the
reading power when the magneto-optical recording medium according to the
examples of the present invention is reading;
FIG. 26 is a graph illustrating the effect of the radiation layer of the
magneto-optical recording medium according to a preferred embodiment of
the present invention;
FIG. 27 shows the injection molding conditions of the transparent
polycarbonate substrate of the magneto-optical recording media according
to a preferred embodiment of the present invention;
FIG. 28 shows the characteristics of the transparent polycarbonate
substrate of the magneto-optical recording medium according to a preferred
embodiment of the present invention;
FIG. 29 shows the formation conditions of an Al film according to a
preferred embodiment of the present invention;
FIG. 30 shows the formation conditions of a recording layer according to a
preferred embodiment of the present invention;
FIG. 31 shows the formation conditions of a reading layer according to a
preferred embodiment of the present invention;
FIG. 32 shows the recording conditions for recording onto the
magneto-optical recording medium according to a preferred embodiment of
the present invention;
FIG. 33 shows the reading conditions for reading from the magneto-optical
recording medium according to a preferred embodiment of the present
invention;
FIG. 34 shows the formation conditions of the SiN film according to a
preferred embodiment of the present invention;
FIG. 35 shows the relations between Al film thickness and the reading
resolution of examples according to a preferred embodiment of the present
invention;
FIG. 36 is a cross-sectional view of a magneto-optical recording medium
according to the ninteenth example;
FIG. 37 is a graph illustrating the characteristics of a recording layer
according to the nineenth example;
FIG. 38 is a schematic illustrating the ninteenth example reading;
FIG. 39 is a schematic illustrating the ninteenth example reading;
FIG. 40 is a graph illustrating the CNR of the reading signal for the
external magnetic field according to the sixth and eighth examples;
FIG. 41 is a graph illustrating the CNR of the reading signal for the
reading laser power according to the eighth and ninth examples;
FIG. 42 is a graph illustrating the CNR of the reading signal for the
recording laser power according to the ninth and tenth examples;
FIG. 43 is a graph illustrating the CNR of the reading signal for the
reading laser power according to the tenth and eleventh examples;
FIG. 44 is a graph for use in comparing the surface smoothness by changing
the etching power in the etching processing to the surface of the
interference layer;
FIG. 45 is a graph illustrating the CNR of the reading signal for domain
length according to the eleventh and twelfth examples;
FIG. 46 is a graph illustrating the measurements of the CNR of the reading
signal for the thickness of an Al radiation layer when the Al radiation
layer is provided on the protection layer of the magneto-optical recording
medium according to the twelfth example;
FIG. 47 is a graph illustrating the CNR of the reading signal for the
reading laser power according to the eleventh and fourteenth examples;
FIG. 48 is a graph illustrating the temperature characteristics of the Kerr
rotation angle; and
FIG. 49 shows the film formation conditions of the recording layer, reading
layer and on layer according to the ninteenth example.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(1) First Example
FIG. 1 is a schematic illustrating the cross-sectional structure of a
magneto-optical recording medium according to the first and second
examples.
The magneto-optical recording medium in FIG. 1 includes a polycarbonate
(PC) substrate 11, and an interference layer 12 about 800 .ANG. thick made
of SiN, a reading layer 13 about 500 .ANG. thick made of GdFeCo, a
recording layer 14 about 500 .ANG. thick made of TbFeCo, and an
oxidation-preventing layer 15 about 800 .ANG. thick made of SiN, all
deposited in this order on the PC substrate 11, and an ultraviolet-setting
plastic layer (not illustrated) is further provided on top of the
oxidation-preventing layer 15 to a thickness of approximately 20 .mu.m as
a protection layer. Each layer illustrated here can be formed by
conventional and well-known sputtering methods or the like.
The composition of the reading layer 13 is "Gd:Fe:Co=31:47:22 atomic
percent (at %)," while the composition of the recording layer 14 is
"Tb:Fe:Co=26:66:8 at %." The results of measurements of the remanence Kerr
rotation angles of the recording layer 14 and reading layer 13 are
illustrated in FIG. 2 with temperature taken as the abscissa. As
illustrated in FIG. 2, the temperature at which the reading layer 13
becomes a perpendicular magnetization film is about 140.degree. C., and
the Curie temperature of the reading layer 13 is about 300.degree. C. The
Curie temperature of the recording layer 14 is about 230.degree. C., and
the compensation temperature of the recording layer 14 is about room
temperature. Reading is achieved by raising the temperature of the target
domain of the reading layer 13 to a copying temperature of about
140.degree. C. as described above, and copying the direction of
magnetization of the same domain of the recording layer 14 to the reading
layer 13 and reading the same domain from the reading layer 13.
In recording, as illustrated in FIG. 3, the CNR saturates when the laser
power is 3.5 mW or more, and when this happens, the reading layer 13 is at
the Curie temperature or higher. For this reason, as illustrated in FIG.
4, recording can be performed when a low external magnetic field such as
50 Oe is applied, and at 200 Oe or more, the CNR is saturated. Considering
that conventionally the application of an external application magnetic
field of 500 Oe or more has been necessary for recording (Optical Data
Storage 1994, Technical Digest Series, vol. 10, pp. 128-129), it can be
understood that the magneto-optical recording medium of this first example
can be recorded by applying an extremely small external magnetic field.
(2) Second Example
The cross-sectional view of the second example is the same as that of the
first example. The second example differs from the first example in that
the composition of the reading layer 13 is "Gd:Fe:Co=31:44:25 at %" and
the composition of the recording layer 14 is "Tb:Fe:Co=26:59:15 at %."
The temperature characteristics of the remanence Kerr rotation angles of
the recording layer 14 and reading layer 13 of the above-described
compositions for the second example are illustrated in FIG. 5. As
illustrated in FIG. 5, the temperature at which the reading layer 13
becomes a perpendicular magnetization film is about 140.degree. C. and the
Curie temperature of the reading layer 13 is about 320.degree. C. On the
other hand, the Curie temperature of the recording layer 14 is about
290.degree. C., and the compensation temperature of the recording layer 14
is about room temperature. This means that the difference in Curie
temperatures between the recording layer 14 and the reading layer 13 is as
small as about 30.degree. C. For this reason, when the temperature falls
below the Curie temperature of the recording layer 14 in the temperature
lowering process during recording, since the temperature is sufficiently
higher than the copying temperature of the reading layer 13, the
magnetized direction of the reading layer 13 is still perpendicular. Thus,
in the reading layer 13, there is no portion where the magnetized
direction is the in-plane direction. Therefore, the direction of the
magnetization to be copied from the reading layer 13 into the recording
layer 14 is also perpendicular, and therefore the CNR is acceptable.
In the case of the second example, as illustrated as ›B! in FIG. 6,
recording can be performed when a low external magnetic field such as 100
Oe is applied, and the CNR is saturated at a CNR of 250 Oe or more. In
FIG. 6, ›A! refers to the characteristics of a conventional
magneto-optical recording medium with the reading layer 13 being composed
of "Gd:Fe:Co=31:34:35 at %" and having a Curie temperature of about
360.degree. C. and the recording layer 14 being composed of
"Tb:Fe:Co=26:66:8 at %" and having a Curie temperature of about
230.degree. C. As illustrated in FIG. 6, in the case of the conventional
magneto-optical recording medium, recording can be performed when an
external magnetic field of 500 Oe or more is applied. Compared with this
conventional magneto-optical recording medium, it is understood that the
magneto-optical recording medium according to the second example can be
recorded by applying an extremely small external magnetic field, much
smaller than the external magnetic field needed for the conventional
magneto-optical recording medium.
(3) Third Example
FIG. 7 is a schematic illustrating the cross-sectional structure of the
magneto-optical recording medium according to the third example.
The magneto-optical recording medium illustrated in this figure includes a
polycarbonate (PC) substrate 21 and a high refraction layer 21 about 800
.ANG. thick made of SiN, a reading layer 23 about 500 .ANG. thick made of
GdFeCo, a recording layer 24 about 500 .ANG. thick made of TbFeCo, an
oxidation-preventing layer 25 about 800 .ANG. thick made of SiN, and a
radiation layer 26 about 200 .ANG. thick made of Al, all deposited on the
substrate 21 in this order, and a protection layer 27 approximately 20
.mu.m thick made of ultraviolet-setting plastic is further provided on top
of the radiation layer 26. Each layer, other than the protection layer 27,
can be formed by conventional and well-known sputtering methods or the
like.
In addition to the above examples, other magneto-optical recording media
variations were manufactured by varying the film thickness of the Al
radiation layer 26 from about 200 .ANG. to about 300 .ANG., 400 .ANG. and
800 .ANG.. Since the exchange-coupled magnetic layer, which includes the
recording layer 24 and the reading layer 23, has a total film thickness of
about 1000 .ANG., any of these magneto-optical recording media is of a
type that does not sufficiently transmit the laser beam, i.e., that
absorbs the reflected laser beam from the magnetic layer. While reading,
the direction of the magnetization of the target domain of the recording
layer 24 is copied into the reading layer 23.
In each of the magneto-optical recording media described above, when the
temperature of a target domain of the reading layer 23 is raised by
focusing the laser spot onto the target domain through the substrate 21,
the direction of the magnetization of a corresponding domain of the
recording layer 24 is copied into an area where the temperature of the
reading layer 23 has exceeded its copying temperature (about 140.degree.
C. for the third example and the variations). By making use of this
phenomenon, the information is read from the recording layer 24. Here,
however, as the temperature at which the spontaneous magnetization of the
recording layer 24 is demagnetized is about 250.degree. C., the
information in the recording layer 24 is retained at the copying
temperature described above.
Furthermore, in the third example and the variations, the copying
temperature of about 140.degree. C. of the reading layer 23 and the
temperature of about 400.degree. C. at which the spontaneous magnetization
direction of the reading layer 23 is demagnetized are achieved by setting
the composition to "Gd:FeCo=32:68 at %," and the temperature of about
250.degree. C. at which the spontaneous magnetization direction of the
recording layer 24 is demagnetized is achieved by setting the composition
to "Tb:FeCo=25:75 at %."
The recorded information was read from each of the magneto-optical
recording media (where only the thickness of the radiation layer 26
differs between each variation as about 200 .ANG., 300 .ANG., 400 .ANG.
and 800 .ANG., respectively), the CNR of each magneto-optical recording
medium was measured, and the results were compared with a recording medium
without the radiation layer 26 shown in FIG. 7, and each of the
magneto-optical recording media of the third example and the variations
was proven to be better than the recording medium without the radiation
layer 26 shown in FIG. 7. Then, in comparisons among the magneto-optical
recording media of the third example and the variations, that with about a
300 .ANG. thick radiation layer 26 was better than that with about a 200
.ANG. thick radiation layer 26, but those with about 300 .ANG., 400 .ANG.
and 800 .ANG. thick radiation layers 26 were proven to be almost the same.
From these results, it is understood that the thickness of the Al
radiation layer 26 is acceptable at about 200 .ANG. but is preferable at
about 300 .ANG. or more.
FIG. 8 is a graph illustrating the CNR of the reading signal of the
magneto-optical recording medium according to one of the third example
variations (where the thickness of the radiation layer 26 is about 400
.ANG.) and the magneto-optical recording medium without the radiation
layer 26 shown in FIG. 7, with the recording domain length taken as the
abscissa. As illustrated in FIG. 8, it is understood that improvements are
conspicuous when the recording domain lengths are about 0.8 .mu.m or more
and about 0.4 .mu.m or less. This is presumably because the heat of the
reading layer 23 flows to the radiation layer 26, thereby improving the
temperature distribution, and, as a result, noise due to the disorder of
the direction of the magnetization, or excessive reading from the
circumferential area of the laser spot (where the temperature is low), is
reduced.
In the structure of the third example and the variations illustrated in
FIG. 7, two or more recording media were prepared by changing the
composition ratio of the reading layer 23 from Gd=30 at % (the copying
temperature in this case is approximately 70.degree. C.) to Gd=33 at %
(the copying temperature in this case is approximately 160.degree. C.),
and the CNR was measured in substantially the same way as described above,
and substantially the same results were obtained.
Furthermore, instead of Al, other materials such as Ag, Cu, Au, W and Mg,
having a high thermal conductivity, were used, and similar results to the
above-given results were obtained.
(4) Fourth Example
FIG. 9 is a schematic illustrating the cross-sectional structure of a
magneto-optical recording medium according to the fourth example.
The magneto-optical recording medium according to the fourth example is
different from that of the third example and the variations only in that a
magnetic layer 28, which is an in-plane magnetization film, made of NiO
about 500 .ANG. thick, is provided between a high refraction layer 22 made
of SiN and a reading layer 23 made of GdFeCo; any other part of the
structure is the same as the corresponding part of the structure of the
third example or the variations. Here, the radiation layer 26 is made of
Al with a thickness of about 400 .ANG..
The in-plane magnetization film magnetic layer 28 is a layer in which the
direction of the magnetization is in parallel with the layer surface
within a range from about room temperature to about the Neel temperature
(about 100.degree. C. in this example). Furthermore, as the magnetic layer
28 is made of NiO with a thickness of about 500 .ANG., the magnetic layer
28 is sufficiently transparent to transmit the laser beam reflected from
the reading layer 23 to the substrate 21.
This magnetic layer 28 is provided to improve the CNR of the reading signal
by aligning the initial direction of the magnetization of the reading
layer 23 parallel to the magnetic layer 28. That is, the initial direction
of the magnetization of the reading layer 23 cannot completely become the
in-plane direction due to the fact that the recording layer 24 is a
perpendicular magnetization film and is magnetically coupled to the
reading layer 23. However, when the direction of the magnetization of the
reading layer 23 changes from the in-plane direction to the perpendicular
direction, the initial state of the reading layer 23 has an effect on this
process of magnetization direction change. Considering this fact, an
in-plane magnetization film magnetic layer 28 is provided to reduce noise
due to the disordered direction of the magnetization and crosstalk noise
due to excessive reading of the signal from the low-temperature area.
The effect of the in-plane magnetization film magnetic layer 28 described
above can further be heightened by properly selecting the Curie
temperature or the Neel temperature. Here, as a material for the in-plane
magnetization film magnetic layer 28, instead of NiO as described above,
CoNiO, CoO, MnFe, FeCr, FeNi, MnNi, PtCo and PdCo may also be used as
alternative materials.
The CNR of the reading signal of the magneto-optical recording medium
according to the fourth example was measured in substantially the same way
as the measurement of the CNR of the third example and the variations, and
an acceptable CNR was obtained. Then, a recording medium with no radiation
layer 26 in FIG. 9 was prepared, and the CNR was compared with this fourth
example. As a result, the CNR of the recording medium of the fourth
example was better than that of the recording medium with no radiation
layer 26.
(5) Fifth Example
FIG. 10 is a schematic illustrating the cross-sectional structure of a
magneto-optical recording medium according to the fifth example.
The magneto-optical recording medium according to the fifth example is
different from the fourth example only in that the in-plane magnetization
film magnetic layer 28 made of NiO is not provided, and a cut-off
magnetization layer 29 about 300 .ANG. thick made of TbFeCoAl is provided
between a reading layer 23 made of GdFeCo and a recording layer 24 made of
TbFeCo; any other part of the structure is the same as the corresponding
part of the structure of the fourth example. Here, Al about 400 .ANG.
thick is used as a radiation layer 26 in substantially the same way as the
fourth example.
In the case of the cut-off magnetization layer 29 described above, the
temperature at which spontaneous magnetization is demagnetized is about
190.degree. C., which is a temperature set lower than that at which the
spontaneous magnetization of the recording layer 24 is demagnetized. In
this fifth example, the temperature of about 190.degree. C. is achieved by
setting the Al content to about 17 at %.
This cut-off magnetization layer 29 is designed to protect the recording of
the information into the recording layer 24 from the effect of the thermal
magnetic characteristics of the reading layer 23. That is, when the area
whose temperature is raised by the irradiation of the laser spot of
recording power falls to about 250.degree. C. (the temperature at which
the spontaneous magnetization of the recording layer 24 is demagnetized)
or less in the temperature reducing process, as the magnetization of the
cut-off magnetization layer 29 is about 0 at this temperature of about
250.degree. C., the magnetized direction of the recording layer 24 is in
the direction of the external magnetic field independently from the
reading layer 23. When the temperature further falls to about 190.degree.
C. (the temperature at which the spontaneous magnetization of the cut-off
magnetization layer 29 is demagnetized) or less, the direction of the
magnetization of the cut-off magnetization layer 29 follows the direction
of the magnetization of the recording layer 24. Therefore, when the
copying temperature during reading is a little more than about 140.degree.
C., which is under about 190.degree. C., the cutoff magnetization layer 29
behaves in substantially the same way as the recording layer 24. Here, as
a material for the cut-off magnetization layer 29, in addition to TbFeCoAl
as described above, TbFeCoNb, TbFeCoCr and TbFeCoNi may also be used as an
alternative material.
The CNR of the reading signal of the magneto-optical recording medium
according to the fifth example was measured in substantially the same way
as the measurement of the CNR of the third example and the variations, and
an acceptable CNR was obtained. Then, a recording medium with no radiation
layer 26 in FIG. 10 was prepared, and the CNR was compared with this fifth
example. As a result, the CNR of the recording medium of the fifth example
was better than that of the recording medium with no radiation layer 26.
(6) Sixth Example-Fourteenth Example
Now, the sixth through fourteenth examples will be described. The
cross-sectional structure of magneto-optical recording media according to
the sixth through fourteenth examples is illustrated in FIG. 11.
Specifically, on a polycarbonate (PC) transparent substrate 1 an
interference layer 2 made of SiN, a reading layer 3 made of GdFeCo, a
recording layer 4 made of TbFeCo and a protection layer 5 made of SiN are
all formed in this order. This structure can be made by conventional and
well-known sputtering methods or the like. In the thirteenth example, on
the protection layer 5, a radiation layer (not illustrated) about 200
.ANG. thick, made of Al, is formed.
(6-1) Composition
The composition of a comparison example and the sixth through fourteenth
examples will be described.
(6-1-1) Example for comparison
In the comparison example, the interference layer 2 is formed to a
thickness of about 800 .ANG., the reading layer 3 is formed to a thickness
of about 500 .ANG., the recording layer 4 is formed to a thickness of
about 500 .ANG. and the protection layer 5 is formed to a thickness of
about 800 .ANG..
The composition of the reading layer 3 is Gd:Fe:Co=23:65.5:11.5 at %. The
composition of the recording layer 4 is Tb:Fe:Co=26:66:8 at %.
(6-1-2) Sixth Example
In the sixth example, the interference layer 2 is formed to a thickness of
about 800 .ANG., the reading layer 3 is formed to a thickness of about 500
.ANG., the recording layer 4 is formed to a thickness of about 500 .ANG.
and the protection layer 5 is formed to a thickness of about 800 .ANG..
The composition of the reading layer 3 is Gd:Fe:Co=31:46:23 at %. The
composition of the recording layer 4 is Tb:Fe:Co=26:66:8 at %.
Thus, the sixth example differs from the comparison example in the
composition of the reading layer 3.
(6-1-3) Seventh Example
The seventh example is substantially the same as the sixth example in the
film thicknesses of the interference layer 2, reading layer 3, recording
layer 4 and protection layer 5, and in the composition of the reading
layer 3 and recording layer 4.
The seventh example differs from the sixth example in that when the
interference layer 2 is formed, the surface of the interference layer 2 is
etched, and then the reading layer 3 is formed.
The etching conditions are a sputtering gas pressure of about 1.2 mTorr for
backward sputtering, a power supply of about 100 W and an etching time of
about 20 min.
(6-1-4) Eighth Example
The eighth example is substantially the same as the sixth example in the
film thicknesses of the reading layer 3, recording layer 4 and protection
layer 5, and in the composition of the reading layer 3 and recording layer
4.
The eighth example differs from the sixth example in that the thickness of
the interference layer 2 is about 700 .ANG..
(6-1-5) Ninth Example
The ninth example is substantially the same as the eighth example in the
film thicknesses of the interference layer 2, recording layer 4 and
protection layer 5, and in the composition of the reading layer 3 and
recording layer 4.
The ninth example differs from the eighth example in that the thickness of
the reading layer 2 is about 1000 .ANG..
(6-1-6) Tenth Example
The tenth example is substantially the same as the ninth example in the
film thicknesses of the interference layer 2, reading layer 3, recording
layer 4 and protection layer 5, and in the composition of the reading
layer 3.
The tenth example differs from the ninth example in that the composition of
the recording layer 4 is Tb:Fe:Co=25:62:13 at %.
(6-1-7) Eleventh Example
The eleventh example is substantially the same as the tenth example in the
film thicknesses of the interference layer 2, reading layer 3, recording
layer 4 and protection layer 5, and in the composition of the recording
layer 4.
The eleventh example differs from the tenth example in that the composition
of the reading layer 3 is Gd:Fe:Co=34:44:22 at %.
(6-1-8) Twelfth Example
The twelfth example is substantially the same as the eleventh example in
the film thicknesses of the interference layer 2, reading layer 3,
recording layer 4 and protection layer 5, and in the composition of the
reading layer 3 and recording layer 4.
The twelfth example differs from the eleventh example in that when the
interference layer 2 is formed, the surface of the interference layer 2 is
etched, and then the reading layer 3 is formed.
The etching power intensity is about 0.05 W/cm.sup.2.
(6-1-9) Thirteenth Example
The thirteenth example is substantially the same as the twelfth example in
the film thicknesses of the interference layer 2, reading layer 3,
recording layer 4 and protection layer 5, and in the composition of the
reading layer 3 and recording layer 4, and in the fact that the surface of
the interference layer 2 is etched by an etching power intensity of about
0.05 W/cm.sup.2 and then the reading layer 3 is formed.
The thirteenth example differs from the twelfth example in that a radiation
layer made of Al with a thickness of about 200 .ANG. is formed on the
protection layer 5.
(6-1-10) Fourteenth Example
The fourteenth example is substantially the same as the eleventh example in
the film thicknesses of the interference layer 2, reading layer 3,
recording layer 4 and protection layer 5, and in the composition of the
reading layer 3 and recording layer 4.
The fourteenth example differs from the eleventh example in that the
sputtering gas pressure for the formation of the reading layer 3 is about
3.5 mTorr while the sputtering gas pressure for the formation of the
reading layer 3 of the eleventh example is about 7 mTorr.
(6-2) Characteristics
Now, various characteristics of the magneto-optical recording media
according to the above-described comparison example and the sixth through
fourteenth examples will be described.
(6-2-1) Sixth Example to Comparison Example
FIG. 12 is a temperature characteristic diagram of the Kerr rotation angle
of the comparison example and sixth example.
The temperature at which the reading layer 3 of the magneto-optical
recording medium according to the sixth example changes to a perpendicular
magnetization layer was about 140.degree. C., in other words, the copying
temperature of the reading layer 3 was about 140.degree. C., and the Curie
temperature was about 350.degree. C., while the Curie temperature of the
comparison example was about 300.degree. C.
When the laser beam spot is focused onto the reading layer 3 through the
substrate 1 and thereby the temperature of the reading layer 3 is raised
to the copying temperature, the direction of the magnetization of the
recording layer 4 is copied into the reading layer 3 in the area that has
exceeded the copying temperature. The copying temperature of the reading
layer 3 of the sixth example is about 140.degree. C. By making use of this
phenomenon, the reading of the information of the recording layer 4 is
performed in the respective magneto-optical recording media according to
the sixth through the fourteenth examples and the comparison example.
FIG. 13 illustrates the Kerr loop around the Curie temperatures of the
sixth example and comparison example.
As illustrated in FIG. 13, the saturation magnetic field of the comparison
example at a temperature of about 280.degree. C. is approximately 500 Oe,
while the saturation magnetic field of the sixth example at a temperature
of about 330.degree. C. is approximately 100 Oe. The magnitude of the
saturation magnetic field at a temperature that is slightly lower than the
Curie temperature relates to the magnitude of the external magnetic field
that is necessary for recording. In other words, the larger the magnitude
of the saturation magnetic field, the larger the magnitude of the external
magnetic field that is necessary for recording. Here, the temperature that
is slightly lower than the Curie temperature is about 330.degree. C. for
the sixth example, and is about 280.degree. C. for the comparison example.
FIG. 14 shows a CNR characteristic of the reading signal of the sixth
example and the comparison example where the recording is performed by
applying a modulated magnetic field. As illustrated in FIG. 14, with the
comparison example, unless an external magnetic field of about 500 Oe or
more was applied, recording was impossible. With the sixth example,
however, the CNR could be saturated by applying as small an external
magnetic field as about .+-.200 Oe or so, and even if the external
magnetic field was as small as about .+-.80 Oe or so, the recording was
still possible.
In the sixth example, where the relative amount of Co as a component of the
reading layer 3 is changed to more than about 50 at %, even when the
temperature is raised, the reading layer 3 is not changed to the
perpendicular magnetization film, and an object of the present invention
can still be achieved.
In the sixth example, although the reading layer 3 is composed of GdFeCo,
even when the reading layer 3 is composed of a four-component material,
such as GdFeCoCr, GdFeCoNi, GdFeCoTi, GdFeCoAl and GdFeCoMn, or a
five-component material, such as GdFeCoNiCr and GdFeCoAlTi, substantially
the same effect as that of the sixth example can be obtained.
(6-2-2) Sixth Example to Seventh Example
FIG. 15 shows plan views of the surfaces of interference layers (ground
layer) 2 of the sixth and seventh examples obtained through an interatomic
force microscope (AFM). From this figure, it is understood that the
surface of the interference layer 2 of the seventh example is smoother
than that of the sixth example. For this reason, the pinning power of the
reading layer 3 and recording layer 4 formed on the surfaces smoothed by
the backward sputtering type etching processing degrades, facilitating the
movement of the magnetic domain walls.
FIG. 16 shows a CNR characteristic of the reading signal of the sixth and
seventh examples where the recording is performed by applying a modulated
magnetic field. From this figure, it is understood that the sixth example
can be recorded from when the external magnetic field is as small as about
.+-.80 Oe, while the seventh example can be recorded from when the
external magnetic field is as still small as about .+-.50 Oe. This is
presumably an effect of forming the reading layer 3 after smoothing the
surface of the ground layer 2 by etching.
(6-2-3) Sixth Example to Eighth Example
FIG. 40 shows a CNR characteristic of the reading signal of the sixth and
eighth examples where the recording is performed by applying a modulated
magnetic field. From this figure, it is understood that the eighth example
can be recorded from when the external magnetic field is as small as about
.+-.80 Oe as can the sixth example, and, in addition to this, the
recording characteristics of the eighth example have been further
improved. This is presumably an effect of forming the interference layer 2
slightly thinner than the interference layer 2 of the sixth example. When
the thickness of the interference layer 2 was within a range of about
600-800 .ANG., acceptable recording characteristics could indeed be
obtained.
(6-2-4) Eighth Example to Ninth Example
FIG. 41 shows a CNR characteristic of the reading signal of the eighth and
ninth examples when the reading laser power is changed. From this figure,
it is understood that the CNR of the ninth example steeply changes at
around the point when the reading laser power is 1.5 mW, which is a better
characteristic than that of the eighth example. This is presumably an
effect of setting the thickness of the reading layer 3 to be about 1000
.ANG., which is thicker than the reading layer 3 of the eighth example. It
was confirmed that such an effect where the CNR of the reading signal
steeply changed at around a certain value of the reading laser power
(about 1.5 mW for the ninth example) could sufficiently be obtained when
the thickness of the reading layer 3 was set to a range of about 800-1200
.ANG..
(6-2-5) Ninth Example to Tenth Example
FIG. 42 shows a CNR characteristic of the reading signal of the ninth and
tenth examples when the recording laser power is changed. Here, the laser
power for the reading is about 1.5 mW. As illustrated in FIG. 42, in the
case of the tenth example in which the relative amount of Co of the
recording layer 4 is different from that of the ninth example, when the
recording laser power is smaller than about 3 mW, which is sufficiently
bigger than the reading laser power of about 1.5 mW, the CNR of the
reading signal sufficiently lowers. In the case of the ninth example,
however, only when the recording laser power is smaller than about 2 mW,
which is a little bigger than the reading laser power of about 1.5 mW, is
the CNR of the reading signal sufficiently lowered. Thus, the possibility
of an adverse effect on the recorded signal, due to irradiation of the
laser beam of the reading power, is smaller in the tenth example than the
ninth example. This is presumably an effect of setting the relative
amounts of Co of the recording layer 4 to be larger in the tenth example
than in the ninth example. Such an effect could indeed be sufficiently
obtained when the component ratio of Co of the recording layer 4 was set
to a range of about 10-16 at %.
(6-2-6) Tenth Example to Eleventh Example
FIG. 43 shows a CNR characteristic of the reading signal of the tenth and
eleventh examples when the reading laser power is changed. From FIG. 43,
it is understood that the CNR of the eleventh example rapidly and steeply
changes at around the point when the reading laser power is about 1.5 mW,
which is a better characteristic than that of the tenth example. This is
presumably an effect of setting the component ratio of Gd of the reading
layer 3 to be about 34 at %, which is larger than the component ratio of
Gd of the reading layer 3 of the tenth example. Such an effect, that the
CNR rapidly and steeply changed at around a certain value of the reading
laser power (e.g., about 1.5 mW for the eleventh example), could indeed be
sufficiently obtained when the composition of Gd of the reading layer 3
was set to a range of about 30-36 at %.
(6-2-7) Etching Power
FIG. 44 is a characteristic diagram comparing the surface smoothness by
changing the etching power when etching is applied to the surface of the
interference layer 2. From FIG. 44, it is understood that when the etching
power intensity is about 0.05 W/cm.sup.2, a desired smoothness can be
obtained by providing etching for about 10 minutes or more.
When the etching power intensity was set to a range of about 0.02-0.08
W/cm.sup.2, a desired smoothness could indeed be obtained.
(6-2-8) Eleventh Example to Twelfth Example
FIG. 45 is a characteristic diagram illustrating the CNR of the reading
signal to the recording domain length for the eleventh and twelfth
examples. As illustrated in FIG. 45, for the twelfth example, even when
the recording domain length was shorter, an acceptable CNR could be
obtained. This is presumably because the smoothness of the surface of the
interference layer 2 by etching facilitated the movement of the magnetic
domain walls, forming more stable domains compared with the eleventh
example.
(6-2-9) Thirteenth Example
FIG. 46 is a characteristic diagram illustrating the measurements of the
CNR of the reading signal to the thickness of an Al radiation layer. Each
of the Al radiation layers was provided on the protection layer 5 of the
magneto-optical recording medium according to the twelfth example,
respectively. Here, the recording domain lengths were set to about 0.5
.mu.m and about 1.5 .mu.m. From FIG. 46, it is understood that when the
thickness of the Al radiation layer is set to a range of about 200-500
.ANG., an acceptable characteristic can be obtained. The radiation layer
about 200 .ANG. thick corresponds to the thirteenth example.
(6-2-10) Eleventh Example to Fourteenth Example
FIG. 47 shows a CNR characteristic of the reading signal of the eleventh
and fourteenth examples when the reading laser power is changed. From FIG.
47, it is understood that in both examples, the CNR of the reading signal
rapidly and steeply changes at around the point where the reading laser
power is about 1.5 mW and an acceptable characteristic can be obtained.
Such acceptable characteristics could indeed be obtained when the
sputtering gas pressure for forming the reading layer 3 was set to a range
of about 2-7 mTorr.
As described above, the effects of the sixth example can be obtained in the
same or better shape even when the surface of the interference layer 2 is
smoothed by etching with an etching power intensity within a range of
about 0.02-0.08 W/cm.sup.2, the thickness of the interference layer 2 is
changed within a range of about 600-800 .ANG., the thickness of the
reading layer 3 is changed within a range of about 800-1200 .ANG., the
atomic percent of Co as an ingredient of the recording layer 4 is changed
within a range of about 10-16 at %, the atomic percent of Gd as an
ingredient of the reading layer 3 is changed within a range of about 30-36
at %, the Al radiation layer is formed on the protection layer 5 within a
thickness range of about 200-500 .ANG., or the sputtering gas pressure for
forming the reading layer 3 is changed within a range of about 2-7 mTorr.
(7) Fifteenth Examples
In this example, description will be given to the substrate for the
magneto-optical recording medium, recording conditions, and the like, with
reference to the appended drawings and tables.
Magneto-optical disks according to preferred embodiments of the present
invention have a magnetic layer. The magnetic layer comprises a recording
layer and a reading layer. The recording layer is a substantially
perpendicular magnetization film and the reading layer is a substantially
in-plane magnetization film at about room temperature. The recorded
information is read from the magneto-optical disk by irradiating a laser
beam with the reading power onto the reading layer. Thus, the temperature
of the irradiated area of the reading layer is raised by the laser
irradiation, and the direction of the magnetization of the recording layer
is copied to the reading layer, and the copied direction of the
magnetization of the reading layer is read. This type of disk is called a
super-resolution-type magneto-optical recording medium. This
magneto-optical recording medium can be recorded and reproduced at a high
density.
FIG. 17 is a cross-sectional view of a magneto-optical recording medium
according to this example. In the construction thereof, an interference
layer 42 is formed on a transparent polycarbonate (PC) substrate 41, and a
reading layer 43, a recording layer 44, a protection layer 45, a radiation
layer 46 and an ultraviolet-setting plastic layer 47 are deposited on the
interference layer 42 in this order.
The manufacture of the magneto-optical disk of this example will be
described.
The manufacturing process for the magneto-optical disk according to a
preferred embodiment of the present invention is illustrated in FIG. 18.
The transparent polycarbonate (PC) substrate 41 is injection molded, then
SiN is deposited on the transparent polycarbonate substrate 41, and then
the SIN is etched by using a plasma. Following this, the reading layer 43,
the recording layer 44, the protection layer 45, the radiation layer 46
and the ultraviolet-setting plastic layer 47 are deposited one after
another as described above.
In this example, instead of the glass substrate conventionally used as a
substrate for the super-resolution-type magneto-optical recording medium,
polycarbonate (PC) substrate is used. Now, the injection molding of the
transparent polycarbonate (PC) substrate will be described.
The injection molding of the transparent polycarbonate (PC) substrate is
largely dependent on mold temperature 1t, mold clamping pressure 2p, resin
injection velocity 3v, heating cylinder temperature 4t and cooling time
illustrated in FIG. 19. In this embodiment, the track pitches were set to
about 1.4, 1.2, 1.0 and 0.8 .mu.m and the groove to land width ratio was
set to about 1:1. The injection molding was performed under the conditions
of the fifteenth through eighteenth examples illustrated in FIG. 27, i.e.,
the molding temperature within a range of about 118.degree.-125.degree.
C., the mold clamping "force" within a range of about 180-220 kg/cm.sup.2,
the resin injection velocity within a range of about 150-200 mm/s, the
heating cylinder temperature within a range of about
310.degree.-340.degree. C., and the cooling time within a range of about
9-13 sec.
When the copying ratio of the molded substrate was expressed as a ratio of
the depth of the groove of the transparent polycarbonate (PC) substrate to
the depth of the groove of the stamper, high copying ratios of about 90%
or more could be obtained under all the conditions. The surface conditions
of the substrates molded under the respective conditions were measured
through an interatomic force microscope (AFM) and the radius of curvature
at the corner of the groove and land, and the like was calculated for each
example, and the results are shown in FIG. 28.
As a result, the radius of curvature for each track pitch was within a
range of about 35-50 nm, the maximum absolute value of the double
refraction was within a range of about 20-25 nm, and the variation in the
double refraction was within a range of about 8-10 nm, which prove to be
acceptable results. The surface roughness of the molded polycarbonate (PC)
substrate was within a range of about 10-50 nm, which also proves to be an
acceptable result. In particular, for the track pitch of about 1.4 .mu.m,
the radius of curvature was about 35 nm and the maximum absolute double
refraction was about 22 nm, and the variation in the circumferential
direction was about 8 nm, which prove to be acceptable results. Here, the
double refraction was measured by using a He-Ne laser beam with a
wavelength of about 633 nm with a double path.
Next, as the interference layer 2, SiN film was deposited on the injection
molded polycarbonate (PC) substrate to a thickness of about 700 .ANG. by
an RF sputtering method under the conditions illustrated in FIG. 34. Then,
after the deposition of the SiN film, the surface of the SiN film was
smoothed by plasma etching, and then Gd.sub.x Fe.sub.100-x+y) Co.sub.y was
deposited to a thickness of about 1000 .ANG. as the reading layer 43,
Tb.sub.x Fe.sub.100-(x+y) Co.sub.y was deposited to a thickness of about
500 .ANG. as the recording layer 44, SiN was deposited to a thickness of
about 800 .ANG. as the protection layer 45, Al was deposited to a
thickness of about 500 .ANG. as the radiation layer 46, and the
ultraviolet-setting plastic was deposited to a thickness of about 10 .mu.m
as the protection film 47.
For SiN as the interference layer 42, of all the conditions shown in FIG.
34, the RF power of about 500 W and Ar gas pressure of about 5 mTorr are
preferable.
The Gd.sub.x Fe.sub.100-(x+y) Co.sub.y as the reading layer 43 was
deposited by an RF two-element magnetron sputtering method under the
conditions shown in FIG. 31. Of all the conditions shown in FIG. 31, the
RF power of about 70 W for Gd and about 200 W for FeCo, and Ar gas
pressure of about 7 mTorr are preferable. In the composition of the
Gd.sub.x Fe.sub.100-(x+y) Co.sub.y, x was within a range of about 25-35
and y was within the range of about 0-40, which is suitable to the
magneto-optical recording medium according to embodiments of the present
invention. Preferably, however, x should be about 30 and y should be about
40.
The Tb.sub.x Fe.sub.100-(x+y) Co.sub.y as the recording layer 44 was
deposited by an RF magnetron sputtering method under the conditions shown
in FIG. 30. Of all the conditions shown in FIG. 30, the RF power of about
500 W and Ar gas pressure of about 5 mTorr are preferable. In the
composition of the Tb.sub.x Fe.sub.100-(x+y) Co.sub.y, x was within a
range of about 15-35 and y was within the range of about 5-30, which are
suitable to the magneto-optical recording medium according to embodiments
of the present invention. Preferably, x should be about 22.5 and y should
be about 14.5. The SiN as the protection layer 45 was deposited by an RF
magnetron sputtering method under the conditions shown in FIG. 34. Of all
the conditions shown in FIG. 34, the RF power of about 500 W and Ar gas
pressure of about 5 mTorr are preferable.
The Al as the radiation layer 46 was deposited by an RF magnetron
sputtering method with an Al alloy, such as Al-Ti, Al-Mn and Al-Nb, as the
target under the conditions shown in FIG. 29. Of all the conditions shown
in FIG. 29, the RF power of about 800 W and Ar gas pressure of about 5
mTorr are preferable. In this case, the Al deposition rate is about 100
.ANG./min. The radiation layer 6 of this example should not be limited to
Al but Au, Pt, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ag, Sn, Sb and W may
also be used. Furthermore, these elements may be used alone or in the form
of an alloy in any combination.
On the protection layer 45, the ultraviolet-setting plastic film 47 is
formed by an ordinary method, for example, by spin coating.
Now, the recording and reading of the magneto-optical recording medium
manufactured as described above will also be described.
Instead of the conventional recording system using an irradiation with a
laser beam of a constant intensity, as illustrated in FIG. 20, the pulse
modulation method, using pulsed laser beams, as illustrated in FIG. 20,
was employed. FIG. 21 is a block diagram illustrating an exemplary
recording apparatus.
The recording signal is input to a synchronizing pulse generation and phase
delay circuit 200, and is converted into a pulse signal with a duty ratio
or duty cycle of about 50% for synchronization with the recording signal,
and then converted into a pulse signal with a phase delayed by about 0-60
ns. This pulse signal is input to a pulse width change circuit 190, and is
converted into a pulse signal with a duty ratio of about 20-60%, and then
is input to a laser diode driver 180. The laser diode driver 180 turns a
laser diode 150 ON and OFF responsive to the pulse signal changed to have
a predetermined duty ratio, and thereby the pulse laser beam is irradiated
onto the magneto-optical recording medium 110 reflected from a mirror 140
and through an objective lens 210.
The recording signal is input to a magnetic head driver 170. The magnetic
head driver 170 drives the magnetic head 160 responsive to the recording
signal, and thereby the recording signal is recorded into the
magneto-optical recording medium.
In this embodiment, the laser beam is pulse modulated as described above,
and, accordingly, the relations between the external magnetic field
corresponding to the recording signal and the pulsed laser beam are such
that a recording signal is recorded while the laser beam is turned ON
about half the time, as illustrated in FIG. 20. Therefore, as is
qualitatively illustrated in FIG. 22, compared with the conventional
method in which the recording is performed while the laser beam is
irradiated with a constant intensity, the red-hot area of the recording
layer is narrower when recording is performed with a pulsed laser beam. In
FIG. 22, conventional recording is shown as (a) and recording according to
an embodiment of the present invention is shown as (b). The red-hot area
means an area where the temperature is raised to the proper level for
recording. In addition to pulse modulating the laser beam, this effect can
also be obtained by depositing Al as the radiation layer on the recording
layer. Furthermore, by forming the radiation layer and pulse modulating
the laser beam, the effect described above becomes even more evident, and
the red-hot area becomes even narrower. This case is shown as (c) in FIG.
22.
In this embodiment the recording into the magneto-optical recording medium
was performed under the conditions shown in FIG. 32. The laser wavelength
was about 680 nm, the numerical aperture of the objective lens was about
0.55, the recording linear velocity was about 2.0 m/sec, and the recording
frequency was about 2.0 MHz, which were all fixed. The external magnetic
field, the recording power and optical pulse duty ratio are preferably, of
all the conditions shown in FIG. 32, about .+-.200 Oe, 6 mW and 40%,
respectively.
The reading of the magneto-optical recording medium that had been subjected
to high-density recording, with a domain length of about 0.5 .mu.m, by
forming the radiation layer and pulse modulating the laser beam, was
performed under the conditions shown in FIG. 33. The laser wavelength was
about 680 nm, the numerical aperture of the object lens 210 was about
0.55, and the reading linear velocity was about 2.0(.+-.0.1) m/sec, which
were all fixed. The reading power of about 2.0 mW or more is preferable of
all the conditions shown in FIG. 33. The reading power of about 2.0 mW or
more was selected as a reading power that enabled a high CNR in the
relations between the reading power shown in FIG. 25 and the CNR in the
reading. Thus, according to FIG. 25, the CNR rises as the reading power
rises, and an almost constant CNR of about 42-44 dB can be obtained when
the reading power is about 2.0 mW or more. Based on this, the laser power
of about 2.0 mW or more was selected as a reading power that enabled a
high CNR. Furthermore, at different reading linear velocities, acceptable
reading powers were selected in the same way. As a result, it was found
that the reading power within a range of about 1.5-2.2 mW is suitable to
the reading linear velocity within a range of about 1.1-1.3 m/sec, the
reading power within a range of about 1.8-2.7 mW is suitable to the
reading linear velocity within a range of about 1.5-1.7 m/sec, the reading
power within a range of about 2.4-3.7 mW is suitable to the reading linear
velocity within a range of about 2.9-3.1 m/sec, the reading power within a
range of about 3.2-4.5 mW is suitable to the reading linear velocity
within a range of about 4.9-5.1 m/sec, and the reading power within a
range of about 4.0-6.0 mW is suitable to the reading linear velocity
within a range of about 8.9-9.1 m/sec.
These recording conditions are also suitable to the magneto-optical
recording media of the first through fourteenth examples.
FIGS. 23 and 24 illustrate the reading characteristics of the
magneto-optical recording medium for high-density recording. FIG. 23
illustrates the relations between the phase difference (phase difference
between the pulse magnetic field and pulse modulated laser beam) in
recording and the CNR in reading when the laser wavelength is about 680
nm, the numerical aperture of the objective lens 210 is about 0.55, the
pulse width of the pulse magnetic field is about 500 nsec, and the number
of pulses of the pulse modulated laser beam is 4. As a parameter, the
laser power was changed from about 5.0 to about 5.5, to about 6.0 and to
about 6.5 mW. With the phase difference within a range of about 0-33 nsec
during recording, when the recording laser power increases from about 5.0
mW to about 5.5 mW, the CNR sharply increases from about 0 to about 37-40
dB, and when the recording laser power increases from about 5.5 mW to
about 6.5 mW, the CNR slowly increases. When the recording laser power was
about 6.5 mW, a CNR of approximately 43 dB was obtained.
FIG. 24 illustrates the relations between the phase difference (phase
difference between the pulse magnetic field and pulse modulated laser
beam) in recording and the CNR in reading when the laser wavelength is
about 680 nm, the numerical aperture of the objective lens 210 is about
0.55, the pulse width of the pulse magnetic field is about 500 nsec, and
the number of pulses of the pulse modulated laser beam is 2. As a
parameter, the laser power was changed from about 4.5 to about 5.0, to
about 5.5 and to about 6.0 mW. With the phase difference within a range of
about 0-60 nsec during recording, when the recording laser power increases
from about 4.5 mW to about 5.0 mW, the CNR rapidly and steeply increases
from about 0 to approximately 35 dB, and when the recording laser power
increases from about 5.0 mW to about 6.0 mW, the CNR slowly increases.
When the recording laser power was about 6.0 mW, a CNR of approximately 45
dB was obtained.
By comparing FIG. 23 and 24, FIG. 24 shows that, by reducing the number of
pulses of the laser beam in recording from 4 to 2, the recording laser
power can be reduced from about 5.5 mW to about 5.0 mW, which indicates an
acceptable CNR.
The effect of the radiation layer 46 in reading is evident from the
comparison between the prior art and embodiments of the present invention
in the relations between the CNR and the domain length illustrated in FIG.
26. Specifically, when the domain length is within a range of about
0.4-1.5 .mu.m, the CNR in reading rises by approximately 1-3 dB. As the
improvement in the CNR is more evident when the domain length is shorter,
even when the domain length is 0.4 .mu.m or less, similar results can be
obtained. Furthermore, for the reading of the magneto-optical recording
medium with a short domain length (i.e., a high density), it was found to
be effective to form the radiation layer 46 on the recording layer 44.
Moreover, the thickness of Al as the radiation layer 46 should not be
limited to about 500 .ANG., but may be within a range of about 200-1000
.ANG.. This range of film thickness was determined from the relations
between the Al thickness and reading resolution shown in FIG. 35, i.e.,
from the fact that the reading resolution rose as the Al thickness
increased and the reading resolution became constant when the Al thickness
was about 200 .ANG. or more.
(8) Nineteenth Example
In this example, a description will be given of the recording layer 34 of
the magneto-optical recording medium by referring to FIGS. 36 through 39
and FIG. 49.
FIG. 36 illustrates a cross-sectional structure of a magneto-optical
recording medium of this example. This example was manufactured by using
the following procedures. On a polycarbonate (PC) substrate 31 a SiN layer
32 with a thickness of about 800 .ANG. was formed by sputtering as a
protection film and also for optical enhancement, like an ordinary
magneto-optical disk. Furthermore, on the SiN layer 32 a Gd.sub.30
Fe.sub.55 Co.sub.15 layer 33 with a thickness of about 500 .ANG. and a
(Mn.sub.80 Cr.sub.20).sub.2 Sb layer 34 with a thickness of about 1000
.ANG. were formed by sputtering. In this sputtering, a complex target
composed of Cr and Sb both mounted on a Mn chip was used. Then, SiN was
formed on the (Mn.sub.80 Cr.sub.20).sub.2 Sb layer 34 to a thickness of
about 800 .ANG. as a protection layer 35, and further an
ultraviolet-setting plastic 36 with a thickness of about 10 .mu.m was
formed by spin coating.
The sputtering conditions for these layers are shown in FIG. 49. Of all the
conditions in FIG. 49, it is the most suitable for the formation of the
SiN layer 32 when the Ar gas pressure is about 0.4 Pa and the power supply
is about 300 W, for the formation of the Gd.sub.30 Fe.sub.55 Co.sub.15
layer 33 when the Ar gas pressure is about 0.67 Pa and the power supply is
about 400 W, for the formation of the (Mn.sub.80 Cr.sub.20).sub.2 Sb layer
34 when the Ar pressure is about 0.67 Pa and the power supply is about 350
W, and for the formation of the SiN layer 35 when the Ar gas pressure is
about 0.4 Pa and the power supply is about 300 W, respectively. The
ultraviolet-setting plastic 36 was spin coated only with a dropping
quantity of about 5 cc, a spin condition of about 100 rpm and about 2 sec
for medium velocity and about 900 rpm and about 3 sec for high velocity,
and a exposure time of about 5 sec with a halogen lamp of about 1 kW. The
(Mn.sub.80 Cr.sub.20).sub.2 Sb layer 34 formed as described above is a
magnetic film having a transition from antiferromagnetism to
ferromagnetism, and the Gd.sub.30 Fe.sub.55 Co.sub.15 layer 33 formed as
described above is a in-plane magnetic film at about room temperature.
FIG. 37 illustrates the relation between the magnetization of the
(Mn.sub.80 Cr.sub.20).sub.2 Sb layer 34 and its temperature by using the
Cr concentration as a parameter. From FIG. 37, it is understood that by
increasing the Cr concentration, the transition point of the (Mn.sub.80
Cr.sub.20).sub.2 Sb layer 34 from antiferromagnetism to ferromagnetism
shifts to higher temperatures, and, after the transition, the
magnetization steeply increases. As the rise of this magnetization is
steeper than that of the prior art, the (Mn.sub.80 Cr.sub.20).sub.2 Sb
layer 34 has a clear copying temperature within a temperature range of
about 40.degree.-200.degree. C. and, therefore, is suitable to be a
material for the magneto-optical recording medium utilizing MSR
technology.
As a result of checking the Curie temperature of the (Mn.sub.80
Cr.sub.20).sub.2 Sb layer 34, it was found that the Curie temperature was
constantly around about 230.degree. C., and the recording of the
information was performed by heating the medium to about 230.degree. C. or
more, using a laser beam of about 780 nm in wavelength, and at a track
pitch of about 1.6 .mu.m and a recording linear velocity of about 5 m/sec.
In reading, since the Curie temperature of the (Mn.sub.80 Cr.sub.20).sub.2
Sb layer 34 is about 230.degree. C., it is preferable that the temperature
should be around about 100.degree. C. In an embodiment of the present
invention, the Cr concentration was set to about 20 at %. As illustrated
in FIG. 38, when a reading beam 66 is irradiated into the magneto-optical
recording medium, a recording layer 62 is heated, the transition from
antiferromagnetism to ferromagnetism is caused within the heated area 65,
and magnetization occurs. When the magnetization occurs within the
recording layer 62, the magnetization is not copied into a reading layer
61, which is an in-plane magnetization film, but the reading layer 61
holds the state of the in-plane magnetization film in this area and
functions as a mask. Therefore, the information of only the heated area 65
is reproduced, and the reading in an area smaller than the irradiated beam
diameter, i.e., MSR reading, is possible.
It was found that when the reading laser power was about 1.5 mW or more,
the reading signal rapidly and steeply appears and MSR reading was
enabled. Furthermore, when the reading laser power was about 2.5 mW, the
CNR of the domain length of about 0.3 .mu.m was about 40 dB. Accordingly,
the information of the recording layer is clearly copied into the reading
layer at about 100.degree. C. or more, and there is no magnetic effect of
the (Mn.sub.80 Cr.sub.20).sub.2 Sb layer 34 on the Gd.sub.30 Fe.sub.55
Co.sub.15 layer 33 within any area other than the reading area. Therefore,
the mask effect can further be improved, the reading noise can be reduced,
the MSR effect can be increased, and highly uniform MSR reading is thus
possible.
Furthermore, according to embodiments of this invention, as illustrated in
FIG. 37, when the Cr concentration of (Mn.sub.80 Cr.sub.20).sub.2 Sb is
within a range of about 10-30 at %, the transition from antiferromagnetism
to ferromagnetism is clearly caused. Therefore, by using (Mn.sub.80
Cr.sub.20).sub.2 Sb having a Cr concentration within this range as the
recording layer, substantially the same MSR reading as that described
above can be achieved.
Moreover, in embodiments of the present invention, Gd.sub.30 Fe.sub.55
Co.sub.15, which is an in-plane magnetization film at about room
temperature, is used as a reading layer. However, the material should not
be limited to Gd.sub.30 Fe.sub.55 Co.sub.15, but any material that can
copy the magnetization of the recording layer may be used. For example, if
an initialization magnetic field, which aligns the magnetization direction
of the reading layer, is used, it is possible to use a perpendicular
magnetization film, such as TbFe, GdCo, TbCo and TbFeCo. When such
material is used, as illustrated in FIG. 39, an initialization magnetic
field 70 is applied and thereby the magnetization of a reading layer 61'
is oriented to a recording layer 62', aligning the magnetization of the
reading layer 61', and then a laser beam 66' is irradiated into the
magneto-optical recording medium for reading.
In a high-temperature area 65', as the magnetization occurs within the
recording layer 62', the magnetization oriented to the recording layer 62'
reverses the direction to be the same as that of the magnetization of the
recording layer 62', and thereby the information of the recording layer
62' is copied into the reading layer 61'. Therefore, it is possible to
reproduce the information only within the high-temperature area 65'. In
addition, if a perpendicular magnetization film with a coercive force of
about 1 kOe or less is used, the magnetic domain to be copied is
demagnetized, and the mask is formed also behind the laser beam, and MSR
reading is thus possible.
On the other hand, if the recording layer has a primary transition point,
the material for the recording layer should not be limited to (Mn.sub.80
Cr.sub.20).sub.2 Sb, but any magnetic material with Mn.sub.2 Sb plus V,
Co, Cu, Zn, Ge or As may also be used. The compositions of materials that
show advantageous results are (Mn.sub.93 V.sub.7).sub.2 Sb, (Mn.sub.75
Co.sub.25).sub.2 Sb, (Mn.sub.90 Cu.sub.10).sub.2 Sb, (Mn.sub.90
Zn.sub.10).sub.2 Sb, (Mn.sub.80 Ge.sub.10).sub.2 Sb and (Mn.sub.80
As.sub.20).sub.2 Sb.
FIG. 48 illustrates the dependence of the Kerr rotation angle on the
temperature in the magneto-optical recording media of the examples
described above. Each curve in this figure is proportional to T.sup.c (T:
temperature). From FIG. 48,
1) by setting the thickness of the reading layer to about 1000 .ANG.;
2) by etching the ground layer;
3) by setting the Co composition of the reading layer to about 20 at %; and
4) by setting the sputtering gas pressure to about 3.5 mTorr, each curve
rapidly and steeply rises. The temperature coefficients C of the Kerr
rotational angles obtained for the respective curves are about 8.99, 9.69,
10.9 and 11.0, respectively. Therefore, in recording and reading by using
these magneto-optical recording media, recording and reading with a
density higher than the prior art can be achieved.
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